According to the Congressional Budget Office, between 1979 and 2011, gross median household income, adjusted for inflation, rose from $59,400 to $75,200, or 26.5%. However, once adjusted for household size and looking at taxes from an after-tax perspective, real median household income grew 46%. A lot of the stats about the stagnation of household income ignore changes in household size, which has declined over the years. Two-earner households currently have a medium income of about $88K. Over the past 20 years, one-earner households have grown much more than any other household size. And then there's the churn* in household income: 3/4 of households will make it into the top quintile for at least a year; and just .6% of households in the top 1% stay there for 10 consecutive years. It's less that there's a permanent class of the rich than that the returns to age and education are greater than they used to be, so that the relative income of the older and most educated is a lot higher than it used to be. None of this isn't to say the poor shouldn't be helped. But it's less the case that "the rich are getting richer" than that for any given year, more income is reported at the higher end of the income distribution than it used to be, relative to the rest - but the people up there change a lot from year to year.
Auten, Gerald, Geoffrey Gee, and Nicholas Turner. 2013. "Income Inequality, Mobility, and Turnover at the Top in the US, 1987-2010." American Economic Review, 103(3): 168-72.